ABSTRACT
Theiler 18 described 27 stages of development of the house mouse ranging from the fertilized ovum to the newborn. He related his stages 1 to 22 to Carnegie stages 1 to 23. We recognize some errors in correlating the two staging systems and have reclassified certain of Theiler’s stages in relation to Carnegie stages. This reclassification is based upon the external and internal features of mouse embryos as described by Theiler, 18 Otis and Brent, 19 and unpublished observations by one of us (B.H.J.J.).
